Package should use systemd presets instead of enabling/disabling services

[ziggythehamster]

The spec currently contains these lines:

    systemctl enable systemd-networkd.service
    systemctl enable systemd-resolved.service
    systemctl disable NetworkManager-wait-online.service
    systemctl disable NetworkManager.service

Rather than do this, the package should create a systemd preset file at %{_presetdir}/80-amazon-ec2-net-utils.preset (where %{_presetdir} is defined to be /usr/lib/systemd/system-preset on Enterprise Linux) which looks like this:

enable systemd-networkd.service
enable systemd-resolved.service
disable NetworkManager-wait-online.service
disable NetworkManager.service

The script in the spec would then need to systemctl preset each of those services instead (and probably also systemctl daemon-reload before) since it expects to also start services.

The reason for using presets instead of having the script directly enable/disable services is that it persists the preference/requirement that systemd-networkd is used instead of NetworkManager. If another preset on the system with a lower priority (which may be provided by the OS vendor, a package, or the sysadmin) expresses the preference to enable NetworkManager either directly or indirectly in a preset, then this package will stop working correctly once the preset is applied (which could happen when packages are installed/upgraded or deliberately via systemctl preset-all). Presets are how the Fedora packaging guidelines now recommend doing this (and these guidelines would apply to Enterprise Linux as well). With a preset in place, NetworkManager will not be automatically enabled, and only an explicit enable would cause that to happen.

(Edited: I typed the priority as 98 originally because that's what I had been using all day, but this would have to be <90 since the OS defaults are 90-* and systemd uses whatever comes first and doesn't allow later ones to override it)